So I played marine a few days ago, and ended up having both the glacial sword and the demonic sword. I was surprised when I noticed that I got procs from both the glacial sword (frost nova) and the demonic sword (shock waves).The item description says that orb effects dont stack ... so do they?

Wc3 orb effects are programmed like this: normally it depends on the slot of your item, unless you have a lifesteal item, lifesteal orb effects have the biggest priority and override any other orb effect no matter in which slot.